1. Field of the Invention
The invention generally relates to evaporative emission control systems, and, more particularly, to evaporative emission control systems in which fuel vapor collected in a canister is drawn into an intake passage of an internal combustion engine for disposal in the engine.
2. Description of Related Art
For example, an evaporative emission control system as disclosed in Japanese Laid-open Patent Publication No. 6-58197 is known which includes a canister for collecting fuel vapor generated in a fuel tank, and a purge control valve that communicates the canister with an intake passage of an internal combustion engine as needed. In this system, when the purge control valve is opened, the intake manifold vacuum is fed to the canister so that the fuel vapor collected in the canister is drawn with air into the intake passage. Thus, the known system is able to dispose of fuel vapor generated in the fuel tank without releasing the same to the atmosphere.
According to a method as disclosed in the above-identified patent publication, the system detects an opening failure of the purge control valve (which occurs when the valve is stuck in the open state), and performs control for correcting the air/fuel ratio while stopping learning of the air/fuel ratio when it determines that an opening failure occurs in the purge control valve. The air/fuel ratio correction control includes the steps of estimating the amount of flow of purge gas based on the engine speed and other parameter(s), and correcting the air/fuel ratio based on the estimated purge gas flow, as disclosed in the above-identified publication. With the known control method, the system can correct the air/fuel ratio by some degree in view of an influence of purge gas arising from an opening failure of the purge control valve, and reduce or suppress fluctuations in the air/fuel ratio, which would otherwise occur in the event of the opening failure.
When an opening failure occurs in the purge control valve, a large amount of purge gas constantly flows into the intake passage of the engine. In the meantime, the operating state of the engine varies with time, and the engine may be placed in various situations, such as a situation in which a large amount of intake air is generated, a situation in which only a slight amount of intake air is generated, a situation in which fuel injection should be stopped, and a situation in which the fuel injection amount should be increased. Under these various situations, appropriate air/fuel ratio control cannot be maintained all the time if the air/fuel ratio is merely corrected based on the estimated amount of flow of purge gas as in the known system described above.